Dwell-fatigue behaviour of additively manufactured Ti6242 alloy via LPBF and HIPPING

Abstract

Ambient-temperature, low-cycle dwell fatigue, conventional low-cycle fatigue and creep tests have been performed on additively manufactured and Hot Isostatic Pressed (HIPPED) Ti-6Al-2Sn-4Zr-2Mo alloy. The low-cycle dwell fatigue life compared with the low-cycle fatigue life showed a dwell debit of 5. The factor decrease in the low-cycle dwell fatigue life from the low-cycle fatigue life remain almost same with decreasing peak stress for 120 s dwell time. Key findings indicate that the Laser Powder Bed Fusion process induces an inherent anisotropy and heterogeneity in the microstructure, which, while mitigated by HIPPING, still influences fatigue resistance under dwell loading. The combination of refined microstructure and residual stress relief from HIPPING resulted in improved dwell fatigue performance, though certain microstructural features, such as columnar grains in the as-built condition, contributed to premature crack initiation sites under cyclic loading. The simulated dwell fatigue behaviour based on the Andrade model indicates there is a three-fold increase in the Andrade coefficient with respect to the creep behaviour. The appreciably high dwell sensitivity has been attributed to higher strain rate sensitivity and low strain hardening coefficient which causes significant cyclic softening of the microstructure generated via LPBF + HIPPING of Ti-6242 alloy.